The CO2 concentration and flux over an urban forest of the Namsan site and an urban residential region of the Boramae site in Seoul of Korea for the non-growing period (2-4 March 2011), the growing period (10-12 June 2011) and the late-growing period (22-24 September 2011) were analyzed. The CO2 concentration of two sites had nearly the same diurnal variations with a maximum value during the nighttime and a minimum value during the daytime and the same seasonal variations with a maximum value during the non-growing period (early spring season) and a minimum value during the growing period (summer season). The CO2 flux over the urban forest did not show any typical diurnal variation during the non-growing period, but showed a diurnal variation with a positively small value in the nighttime and a negatively large value in the daytime during the growing and the late-growing periods due to the photosynthesis in the urban forest. The CO2 flux over the urban residential region showed the positive daily mean value for all periods with large values during the non-growing period and small values during the growing period. And it showed a diurnal variation with two maxima at 06-10 LST and at 18-24 LST and two minima at 03-06 LST and 11-15 LST and was strongly correlated with the usage of liquefied natural gas for cooking and heating from surrounding houses.